1. Field of the Invention
The present invention relates to a controller for a power converter, especially to a controller capable of providing a protection signal for a load of a power converter in response to a failure of an AC power.
2. Description of the Related Art
In powering some television apparatuses or portable computers, a power converter is required to provide a power-fault signal along with a DC output voltage for the television apparatuses or portable computers. The power-fault signal is required to be active when a brownout—a drop in voltage—of an AC power occurs or when the AC power gets shut off. As the DC output voltage may still remain at an operable level for a while after the brownout occurs or the AC power gets shut off, if the power-fault signal is not active immediately, the television apparatuses or portable computers may get damaged or malfunction. Please refer to FIG. 1, which illustrates a prior art power converter providing a power-fault signal. As illustrated in FIG. 1, a prior art power converter, including a bridge rectifier 101, a power transmission circuit 102, diodes 103-104, a resistor 105, a diode 106, a capacitor 107, a PWM (pulse width modulation) controller 108, an NMOS transistor 109, a resistor 110, a feedback circuit 111, diodes 112-113, a capacitor 114, resistors 115-116, a shunt regulator 117, a photo coupler 118, a bipolar transistor 119, a resistor 120, and an X capacitor 121, provides an output voltage VO and a power-fault signal VPFLT for a load 130. The load 130 can be a television apparatus or a portable computer.
The bridge rectifier 101 is used to generate an input voltage VIN according to an AC power Vac.
The power transmission circuit 102, including a transformer, a diode, and a capacitor (not shown in the figure), is used to transmit power from the input voltage VIN to a DC output voltage VO and provide a voltage VAUX from an auxiliary coil of the transformer under a control of the NMOS transistor 109.
The diodes 103-104 and the resistor 105 form a start-up circuit to provide power for the PWM controller 108 via a HV pin during an initial period after the AC power Vac is applied.
The diode 106 and the capacitor 107 generate a DC voltage VCC according to VAUX. When the DC voltage VCC is built up, the current path of the start-up circuit will be switched off, and the PWM controller 108 will be solely powered by the DC voltage VCC.
The PWM (pulse width modulation) controller 108 is powered by the DC voltage VCC to provide a gating signal VG in response to a current sensing signal VCS and a feedback signal VFB.
The NMOS transistor 109 switches the power transmission circuit 102 in response to the gating signal VG.
The resistor 110 generates the current sensing signal VCS according to a primary current IP flowing through a primary coil of the transformer when the NMOS transistor 109 is on.
The feedback circuit 111 generates the feedback signal VFB according to a difference of the DC output voltage VO and a reference voltage, the reference voltage being generated in the feedback circuit 111. When in operation, the DC output voltage VO will approach the reference voltage.
The diodes 112-113 and the capacitor 114 are used to generate a line voltage VLINE corresponding to the amplitude of the AC power Vac.
The resistors 115-116 generate a control voltage VX corresponding to a ratio of the line voltage VLINE.
The shunt regulator 117 has a control end connected with the control voltage VX, a cathode connected to the photo coupler 118, and an anode connected to a first ground. When the control voltage VX is higher than a threshold voltage, the shunt regulator 117 will be turned on to pull down the voltage at the cathode to the first ground; and when the control voltage VX is below the threshold voltage, the shunt regulator 117 will be turned off.
The photo coupler 118 has a first terminal coupled to the DC voltage VCC, a second terminal connected to the cathode, a third terminal coupled to the DC output voltage VO, and a fourth terminal connected to a second ground. When the shunt regulator 117 is on, a current IIN will flow through the first terminal and the second terminal to turn on a channel between the third terminal and the fourth terminal.
The bipolar transistor 119 and the resistor 120 form an inverter to provide the power-fault signal VPFLT.
The X capacitor 121 is used to suppress a differential interference accompanying the AC power Vac.
When the control voltage VX is higher than the threshold voltage of the shunt regulator 117, the channel between the third terminal and the fourth terminal of the photo coupler 118 will be formed to generate a low voltage at the third terminal, and the power-fault signal VPFLT will be at a high level to indicate a normal status; and when the control voltage VX is lower than the threshold voltage of the shunt regulator 117, the channel between the third terminal and the fourth terminal of the photo coupler 118 will be off to generate a high voltage at the third terminal, and the power-fault signal VPFLT will be at a low level to indicate a power-fault status.
However, as the X capacitor 121 may still hold charges for as long as 2 seconds after the AC power Vac is switched off, the power-fault signal VPFLT may fail to be active well before the DC output voltage VO falls below a minimum operable level, and the load 130—a television apparatus or portable computer—can therefore get damaged or malfunction. Besides, as the generation of the power-fault signal VPFLT according to the design of FIG. 1 involves a separate circuit, which uses quite a few additional components and occupies an additional board area, the manufacturing cost will be increased substantially.
To solve the foregoing problem, a novel controller is needed.